With the prevalence of an office computer, such as a minicomputer, a personal computer and a work station, a magnetic tape for recording computer data as external storage medium (a so-called backup tape) has been vigorously studied. For the realization of the magnetic tape for such a use, the improvement of recording capacity has been strongly demanded conjointly with the miniaturization of a computer and the increase of information processing ability (e.g., throughput).
Magnetic layers comprising an iron oxide, a Co-modified iron oxide, CrO.sub.2, a ferromagnetic metal powder, or a ferromagnetic hexagonal ferrite powder dispersed in a binder, which are coated on a support, have been conventionally widely used in magnetic recording media. Ferromagnetic metal powders and ferromagnetic hexagonal ferrite powders among these have been known to have excellent high density recording characteristics.
For improving characteristics of a disc-like magnetic recording medium, JP-A-64-84418 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes the use of a vinyl chloride resin having an acidic group, an epoxy group and a hydroxyl group, JP-B-3-12374 (corresponding to U.S. Pat. No. 4,788,092) (the term "JP-B" as used herein means an "examined Japanese patent publication") proposes the use of a metal powder having a coercive force (Hc) of 1,000 Oe or more and a specific surface area of from 25 to 70 m.sup.2 /g, and JP-B-6-28106 proposes to regulate the specific surface area and magnetic susceptibility of magnetic powders and contain an abrasive.
For improving the durability of a disc-like magnetic recording medium, JP-A-54-124716 proposes the use of a nonmagnetic powder having a Mohs' hardness of 6 or more and a higher fatty acid ester, JP-B-7-89407 proposes to regulate the volume of voids containing a lubricant and regulate the surface roughness to from 0.005 to 0.025 .mu.m, JP-B-7-36216 (corresponding to U.S. Pat. No. 4,797,321) proposes the use an abrasive having a particle size of from 1/4 to 3/4 of the magnetic layer thickness and a fatty acid ester having a low melting point, and JP-A-3-203018 (corresponding to U.S. Pat. No. 5,635,294) proposes the use of a ferromagnetic metal powder containing Al and a chromium oxide.
As the constitution of a disc-like magnetic recording medium having a nonmagnetic lower layer and an intermediate layer, JP-A-3-120613 proposes the constitution comprising an electrically conductive layer and a magnetic layer containing a metal powder, JP-A-6-290446 (corresponding to U.S. Pat. No. 5,591,512) proposes the constitution comprising a magnetic layer having a thickness of 1 .mu.m or less and a nonmagnetic layer, JP-A-62-159337 proposes the constitution comprising an intermediate layer comprising a carbon and a magnetic layer containing a lubricant, and JP-A-5-290358 proposes the constitution comprising a nonmagnetic layer in which the carbon size is regulated.
Further, with the prevalence of an office computer, such as a minicomputer and a personal computer, reliability on the use of a magnetic tape for recording computer data as external storage medium (a so-called backup tape) in various environmental conditions due to widening of use environments of magnetic tapes (in particular, under fluctuating temperature/humidity conditions), reliability on data storage, and reliability on performance, such as stable recording/readout of data in multiple running due to repeating use at high speed, have been increasingly demanded in recent years.
Magnetic tapes which are used in digital signal recording systems vary according to each system, for example, magnetic tapes corresponding to a so-called DLT type, 3480, 3490, 3590, QIC, a D8 type and a DDS type are known. In every system, the magnetic tape comprises, on one surface side of a support, a magnetic layer of a single layer structure having a comparatively thick layer thickness, e.g., from 2.0 to 3.0 .mu.m, containing a ferromagnetic powder, a binder and an abrasive, and a back coating layer provided on the surface side of the support opposite to the side having the magnetic layer for purposes of preventing winding disarrangement and maintaining good running durability. However, in general, in a magnetic layer of a single layer structure having a comparatively thick layer thickness as described above, there is a problem of thickness loss which generates the reduction of output.
For the improvement of the reduction of reproduction output due to thickness loss of a magnetic layer, thinning of a magnetic layer has been known. For example, JP-A-5-182178 discloses a magnetic recording medium comprising a support having thereon a lower nonmagnetic layer containing an inorganic powder dispersed in a binder and an upper magnetic layer containing a ferromagnetic powder dispersed in a binder and having a thickness of 1.0 .mu.m or less, which is coated on the lower nonmagnetic layer while the nonmagnetic layer is still wet.
However, with the rapid trend of the increase of the capacity and density of disc-like and tape-like magnetic recording media, it has become difficult to obtain satisfactory characteristics even with these techniques. That is, there is such a problem as output is reduced, an error rate is increased and durability is also decreased particularly with the enhancement of the capacity and density. Therefore, it has become difficult to make the reduction of an error rate compatible with the improvement of durability.